The present invention relates to a treating process for the separation of coated nuclear fuel particles from a graphitic matrix in which the nuclear fuel particles are embedded, as well as to an installation for the conditioning of graphitic fuel elements of high-temperature nuclear reactors.
Nuclear fuel particles are worked out of a graphitic matrix during the reconditioning of spent fuel elements of high-temperature nuclear reactors for the reconditioning of the nuclear fuels. For this purpose, there is required a breaking open of the fuel elements with the subsequent separation of the exposed nuclear fuel particles which contain the nuclear fuels, prior to the particles being chemically dissolved and cleaned. For graphitic fuel elements there must be taken special care that the graphite, which constitutes about 90 to 95% of the quantity of the fuel element, must be removed and the coated nuclear fuel particles which are embedded in the graphitic matrix are isolated. The comminuting of the fuel elements, the separation of the nuclear fuel fraction and graphite fraction, the removal of the graphite mass and a chemical dissolution of the nuclear fuel particles are consequently the process steps which characterize the initial stage of a reconditioning installation for fuel elements. The initial stage is designated as the "head-end" of a reconditioning installation.
2. Discussion of the Prior Art
During the breaking open of the graphitic fuel elements there is aimed at conducting the comminuting step so that subsequently thereafter there becomes possible an extensive quantitative separation between graphite and nuclear fuel particles. The utilization of known comminuting aggregates, such as a hammer mill, jaw crusher, and roll crusher, does, however, deliver an extremely heterogeneous ground product from which only about 60 to 70% of the ground material can be sifted off as nuclear fuel-free graphite; referring to the report GULF-GA-A No. 10 784,1971. Upon the exposure of the graphitic matrix with the integrated nuclear fuel particles in roll crushers, during the subsequent sifting of the ground product, transmitted in an undesired manner are also fragments of the nuclear fuel particles within the separated graphite fraction, which further reduces the economics of the already extremely complex reconditioning process.
The effectuation of a controlled comminuting step in which the graphite is essentially destroyed, and wherein the coated nuclear fuel particles, insofar as they include hard coatings, such as, for example, silicon carbide(SIC) shells, or the nuclear fuel particles themselves are to remain substantially preserved, is also a goal for a head-end stage in which the graphite is burned in a high-frequency vortex furnace, before the nuclear fuel particles are chemically dissolved. However, independently thereof, that during such process step, besides the emission of volatile fission products into the environment, there are encountered additional problems due to the therewith connected emission of radioactive carbon(carbon isotope C-14), through the employment of jaw crushers and subsequently used roll crushers, there is also only produced a ground product which consists of up to 87% of intact nuclear fuel particles, up to 5% of fragments of nuclear fuel particles, and up to 8% of nuclear fuel particles which are still coated with SiC; refer to H. M. Muller, "Head-End Processes for HTGRBE", NUKEM-160, 1973.